Page 312 - Six Sigma Demystified

292 Six SigMa DemystifieD

Use Options\Run Test to apply relevant run-test rules.
Use Options\Format\Show Moving Range Chart to turn on/off moving-range
chart display.

Use Options\Analysis\Curve Fit to change to/from normal and Johnson curve
fits, which are automatically applied to the control limit calculations. This
allows users to see the data in raw units rather than transformed units (as in
Minitab).

Interpretation

Research has shown that for processes following the normal distribution, when
a special cause is detected on the moving-range chart, it also will appear on the
individual-X chart, thus making the moving-range chart redundant.
Never adjust the process based on the observation’s value relative to speci-
fications because this tampering will increase the overall variation in the
process.
If there are any out-of-control points on the individual-X chart, then the
special causes must be eliminated. Brainstorm and conduct designed experi-
ments to find the process elements that contribute to sporadic changes in pro-
cess location. To predict the capability of the process after special causes have
been eliminated, you should remove the out-of-control points from the analy-
sis, which will remove the statistical bias of the out-of-control points by drop-
ping them from the calculations of the centerline and control limits.
Look for obviously nonrandom behavior. Use the run-test rules, which apply

statistical tests for trends to the plotted points.
If the process shows control relative to the statistical limits and run tests for
a sufficient period of time (long enough to see all potential special causes), then
you can analyze its process capability relative to requirements. Capability is
only meaningful when the process is stable because we cannot predict the out-
come of an unstable process.

interaction Plots

Interaction plots are a particular form of point-and-line charts used to graphi-
cally show the relationship among three parameters, usually two factors and a
response.